The present invention relates, in general, to boiler drums and, in particular, to a new and useful system for monitoring the water level of a drum at a pressure column.
Presently, water level indication devices are utilized at a pressure column of a pressure drum of a boiler in order to indicate the presence of water and steam. One device which has been utilized is a multiple electrode remote level indicator (MERLI). MERLI is used as a trademark for a drum gauge sold by the Diamond Power Specialty Company of Lancaster, Ohio.
Existing devices such as the MERLI Drum Gauge have proved to be unreliable during cyclic type conditions. Cyclic boiler operation, often experienced at power plants used for peak power consumption only, results in dynamic water conductivity levels, resulting from a combination in drum pressure and water chemistry. This results from a change in drum pressure. Thus, the higher the normal operation pressure, the more dramatic the conductivity changes.
The MERLI device determines water level of the drum by measuring the difference in conductivity of water versus steam through a set of electrodes which penetrate the side of the MERLI Water Column. Thus, alteration in water conductivity may bias the results. The existing circuitry of the MERLI contains a sensitivity adjustment so that the boiler operator can adjust the unit to the current boiler conditions, i.e. water conductivity. This requires that the MERLI sensitivity be constantly readjusted during start-up and shut-down.
Water and steam from the drum is present within the column and the steam condenses on the internal surface of the column causing a sweating effect. This effect, prevalent in high pressure units, will actually alter conductivity measurements by not allowing the electrodes to adequately dry. As shown in FIG. 1, if this occurs, and the sensitivity between water and steam is relatively close, an electrode located in the presence of steam may appear to be conducting. This is known as water-over-steam fault which is indicated at 10.
Presently, there is no known device operating from a conductivity/resistivity measurement which avoids this problem.